Video information and rendered graphical images are being combined in an increasing number of applications. Examples include animated icons, on-screen menus, video windows in a graphical display, etc. Typically, in these applications the video information is generated separately from the graphical information and the two must be combined before being output to a display device.
In many cases, video information is received in a format with a non-square pixel raster suitable for an expected screen aspect ratio. The aspect ratio is determined based on the ratio between the width of the screen or display area and the height of the screen. In contrast to the video information, graphics rendering systems typically format the graphics information based on a square pixel raster.
In prior art systems that combined separately generated video and graphics display information, the scaling of the video information to match the aspect ratio of the display was based upon the scaling of the graphics information, and the limitations of the graphics scaling controlled the limitations of the video scaling. This technique was suitable for computer graphics displays in which a small window was allotted to video display. In other systems, such as televisions that used closed captioning, graphics scaling systems were not present, and graphics data was rendered to non-square pixel graphics. In this case, the graphics information was limited by the video raster limitations.
Systems in which the video scaling is a subset of the graphics scaling require large amounts of memory to contain both the video information and the graphical information. This is problematic and wasteful in video systems that display or process only a small amount of graphics data. For example, if the video display information uses the entire display screen while the graphics display information requires only a small portion of the display, the amount of memory allotted to the graphics information will need to encompass the entire frame in order to allow the video information to use the entire frame.
Allocating large amounts of memory in the video graphics circuit to graphics information when a smaller amount of memory is adequate wastes both memory storage space and memory bandwidth. The wasted memory bandwidth is especially problematic in video graphics systems that display real time video. In such systems, the demands of the video portion of the display are very demanding upon the memory, and efficient utilization of the memory by the graphics portion of the display is crucial. For example, in the case where an animated icon is superimposed on a video display, the video information requires the entire display, but the graphics information merely requires a small amount of screen space. In prior art scaling systems where the graphics scaling controls the amount of scaling allowed for the video display information, memory corresponding to the entire display would need to be allocated for graphics information. Considering only the small amount of memory is needed to store the limited amount of graphics information, the majority of the memory allocated for graphics information is wasted.
Therefore, a need exists for a video graphics system that allows video information and graphical information to be scaled independently.